This invention relates to a polycrystalline diode and especially relates to a polycrystalline diode formed in a polycrystalline silicon layer mounted on a substrate and being capable of use with both forward bias and reverse bias.
Generally speaking, a diode formed in a polycrystalline silicon layer is easily insulated and isolated from another portion by using an oxide film. Therefore, it is often provided in a device for handling a relatively high electrical voltage such as a power MOS transistor or the like, and is used in a part which must be able to withstand high voltage, such as a surge absorber.
However, when forming a diode in a single crystal silicon layer, a region containing a low concentration of impurities is formed between a P-type region containing a high concentration of impurities and an N-type region in order to be able to withstand a high electrical voltage, and the width of the low concentration is sufficient to obtain a predetermined break down voltage (this width can be determined by a depletion layer extended at that time, though it will be less than 10 .mu.m when the withstand voltage thereof is about several tens of volts.)
The diode thus provided has good performance and a low forward resistance even if used with forward bias.
The reason for this is that when the single crystal silicon is used, as the life time of the carrier is long and thus the injection of the carrier is extended up to a distance of several tens of .mu.m beyond the width of the low concentration region, it is not necessary to limit the width of the low concentration region intentionally, and it is sufficient for the width to be set at less than about 10 .mu.m so that the forward resistance does not become too high.
However, when the diode formed in the polycrystalline silicon layer is used with forward bias, in the polycrystalline silicon layer, the life time of the carrier is extremely short because of scattering or trapping as grain boundaries and accordingly, when the method for obtaining a high withstand voltage used in the single crystal silicon as mentioned above is used in this case without any modification, the low concentration region will become resistant, and the forward resistance of the diode will become extremely high because due to the resistance of a polycrystalline silicon having a low concentration of impurities being remarkably higher than that of the single crystal silicon. For example, in U.S. Pat. No. 4,492,974, a polycrystalline diode having a construction as mentioned above is shown, although when a polycrystalline silicon layer is deposited on a substrate utilizing a standard method for forming a polycrystalline silicon layer in which silicon hydride SiH.sub.4 is thermally decomposed at a deposition temperature of around 600.degree. C. and a pressure of around 50 Pa with a LPCVD device to deposit to form the polycrystalline silicon layer with a thickness about 1000-4000 .ANG. , the grain of the crystal obtained is less than 0.5 .mu.m and the carrier diffusion length is less than 1 .mu.m, even after the annealing treatment is carried out.
Also, in this polycrystalline diode having such a film characteristic and formed in the polycrystalline silicon layer, when the width of the low concentration region is increased in order to obtain a high break down voltage, a condition in which the carrier diffusion length in the low concentration region is reduced to become smaller than the width thereof, will occur and thus the forward resistance thereof will be remarkably increased.
Accordingly, heretofore, a diode in which the P-type region and the N-type region both containing impurities at a high concentration are contacted directly with each other, is formed, and in this case, as the break down voltage thereof will be around 6 V, a plurality of the diodes thus formed can be used in series in order to obtain a high break down voltage.
Even when a diode is formed with the method mentioned above, as the forward resistance of each diode is determined generally with the consideration of a plurality of diodes connected to each other, the overall size thereof will become large and further, the voltage V.sub.F before the forward current starts to flow will be increased leading to the problem of the efficiency thereof being decreased.